LM1872

Manufacturer Part NumberLM1872
DescriptionRadio Control Receiver/Decoder
ManufacturerNational Semiconductor
LM1872 datasheet
 


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Circuit Description
(Continued)
Thus either 3 4 5 or 6 channels are transmitted to repre-
sent the four possible codes that two digital channels repre-
sent The receiver intrinsically counts channels with its de-
coder flip-flops by responding to the negative edges of the
demodulated RF waveform of which there is always one
more than the number of channels The two LSBs of the
binary count are read latched and fed to the output drivers
which comprise digital channels A and B
RECEIVER SECTION
The receiver circuit is a simple single conversion design
with AGC which mixes down to 455 kHz and provides
R1
Motor decoupling
t’
SYNC
R2
Sync timer R2
R2
470k
e
s
0 7 C6
R3
Mixer decoupling
C1
LO bypass optional
C2
LO tank C2
43 pF
27 MHz
e
24 pF
49 MHz
e
C3
Ant input tank C3
39 pF
27 MHz
e
24 pF
49 MHz
e
C4
V
bypass
BIAS
C5
Motor decoupling
t’
SYNC
C6
Sync timer C6
C6
0 5 F
e
s
0 7 R2
C7
Mixer decouple 0 01 F
C7
0 1 F
s
s
C8
AGC
C9
IF bypass optional
C10
V
a
bypass 0 01 F
C10
0 1 F
s
s
FIGURE 5 Typical Application Circuit for 27 MHz or 49 MHz
58 dB of gain using the suggested transformers in Figure 5
The active digital detector provides an additional 30 dB gain
over a silicon diode resulting in an overall system gain of
88 dB More or less gain can be obtained by using different
transformers The frequency range of operation extends
from 50 kHz to 72 MHz encompassing a wide range of allo-
cated frequency bands
The short (1 to 2 ) vertical whip antenna that is typically
used has a very low radiation resistance (0 5
approximately 3 pF to 5 pF of capacitance This antenna is
coupled to the mixer through a high Q tank consisting of C3
C11
LO bypass
L1
LO coil
Toko 10k type (KXNA-4434 DZ) 9T 0 8 H
Toko 10k type (KEN-4028 DZ) 6T 0 4 H
L1 could be made a fixed coil if desired
T1
455 kHz mixer transformer
Toko 10 EZC type (RMC-202313 NO) Qu
e
Pin 1–2 131T pin 2–3 33T
Pin 1–3 164T pin 4–6 5T
T2
455 kHz IF transformer
Toko 10 EZC type (RMC-402503 NO) Qu
e
Pin 1–2 98T pin 2–3 66T
Pin 1–3 164T pin 4–6 8T
T3
Ant input transformer
Toko 10k type (KXNA-4434 DZ) 3T sec
9T pri of 0 8 H
Toko 10k type (KEN-4028 DZ) 1 T sec
X1
3rd overtone parallel-mode crystal
D1
Electrostatic discharge (ESD) protection
7
to 4 ) and
TL H 7912 – 11
27 MHz
49 MHz
110
110
27 MHz
6T pri of 0 4 H
49 MHz
Toko America
1250 Feehanville Drive
Mount Prospect IL 60056
(312) 297-0070